78. Crater Creep

A tall pile of tar will slowly flow downhill, ultimately spreading into a nearly horizontal sheet of tar. Most material, under pressure, “creeps” in this way, although rocks deform very, very slowly.

Calculations show that the growing upward bulges of large crater floors on the Moon should reach their current extent in only 10,000 to 10,000,000 years.a Large, steep-walled craters exist even on Venus and Mercury, where temperatures are hot enough to melt lead. Therefore, creep rates on those planets should be even greater. Most large craters on the Moon, Venus, and Mercury are thought to have formed more than 4,000,000,000 years ago. Because these craters show no sign of “creep,” these bodies seem to be relatively young.